1. Field of the Invention
The present invention relates to a partial doping technology with an impurity, which is necessary for a preparing process of such semiconductor device as MOS (Metal-Oxide-Semiconductor) type or CMOS (Complementary Metal-Oxide-Semiconductor) type semiconductor device. In particular, the present invention provides a doping technology, which is capable of a selective method to dope a different region with a different impurity, using a simple and convenient process, and also which is capable of an efficient doping in a low temperature process.
2. Description of the Related Art
It is indispensable to arrange a process for partially controlling of a resistance rate, by adding an impurity which selectively gives one conductivity type to a part of semiconductor, in case where such semiconductor device as MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor) or CMOS type device is prepared.
In a conventional process, an impurity doping has been carried out by the following method. First of all, a shield film is formed on a surface of semiconductor to keep away the intrusion of impurity. Then, the shield film in the region, where a doping will be effected in accordance with a photolithography process, is removed to form a mask. After that, the doping with a needed impurity is executed by a heat-diffusion method or an ion-implantation method.
However, such doping method in the conventional process as mentioned above creates the following problems.
(1) In case where an impurity is doped in a semiconductor by a heat-diffusion method, there poses a problem that a high temperature process is required. For example of a silicon semiconductor, it is necessary to heat a silicon semiconductor sample at a temperature of 1000 to 1200xc2x0 C., thereby making it difficult to form a shallow impurity layer which is required for a high density IC, and posing a problem of impurity redistribution and defect resulted from the high temperature process.
(2) In case of the impurity doping method by an ion-implantation, there poses the same problem as mentioned in the above (1), because it is in need of a post heat-treatment at a temperature of 600 to 950xc2x0 C., to activate the impurity and to recover the defect.
Also, as a problem in common with the heat-diffusion method and the ion-implantation method stated above, both of them need a high temperature process extremely over 600xc2x0 C. For example, in case of an active matrix type liquid crystal display device to which an attention has been paid recently, since MOS type thin film transistor (TFT) is formed on a glass substrate, it has been difficult to employ the above heat-diffusion method and the ion-implantation method, if a cheap glass substrate which has a heat resistant temperature of 600 to 700xc2x0 C. is used.
Further, in case of a selective doping, it is needed to form a mask as mentioned above. Then, a photolithography process followed by a complicated process will be required, and it has been well known that the photolithography causes a yield to be lowered.
As noted above, there has been a preparing problem that the high temperature process is required in the conventional impurity doping method, and further, a mask forming process, which is in need of a photolithography process for the selective doping, is required. So far, as a doping technology, a heat-diffusion method and an ion-implantation method have been known. In the heat-diffusion method, an impurity will be diffused into a semiconductor at such a high temperature as 1000 to 1200xc2x0 C., and in the ion-implantation method, an ionized impurity will be accelerated in an electric field to be implanted in a prescribed place.
However, the diffusion coefficient of impurity, D is shown as D=Do exp[xe2x88x92Ea/kT], and it is dependent on the absolute temperature T by the exponential function. Here, Do is a diffusion coefficient at T=∞, and k is Boltzman coefficient. Then, it has been preferable to effectively diffuse the impurity into a semiconductor at as high temperature as possible, and it has been generally conducted at a temperature above 1000xc2x0 C., in case of the heat-diffusion method. Also in case of the ion-implantation method, it has been needed for the activation of impurity and the recovery of defect to carry out a post heat-treatment process at a temperature of 600 to 950xc2x0 C.
In recent years, an active matrix type liquid display device, which uses a TFT (Thin Film Transistor) provided on a glass substrate as a switching element of pixel, has been partly put into practical use. But it is common that these form the source, drain region of TFT as an ohmic contact, with one conductive type amorphous silicon. Also, the structure of TFT takes an inverse stagger type, and as a structural problem, it has been prone to generate a parasitic capacity.
Accordingly, the usage of TFT which forms the source, drain region by self-alignment has been investigated. For that purpose, it has been required to employ an ion-implantation method or an ion-shower method. These methods, however, have been in need of the post heat-treatment process at a temperature of 600 to 950xc2x0 C., so as to activate an impurity and recover a defect, as mentioned above, and it has been industrially difficult to use them, considering that a heat resistant temperature of a cheap glass substrate is in the range of 600 to 700xc2x0 C.
To solve such heat damage problem as is given to a glass substrate, a doping technology using a laser beam irradiation has been known. For an example of them, there is a method that a thin film of impurity is formed on the semiconductor surface into which a doping is to be effected, then by an irradiation of laser beam, the thin film of impurity and the semiconductor surface is molten to dissolve the impurity. There is also another method that an impurity is added and diffused into a semiconductor through a gaseous phase, by an irradiation of laser beam toward the semiconductor surface, in an atmosphere of reactive gas containing an impurity to be doped. Particularly, in case of using a pulse irradiation type excimer laser, it has a feature that the temperature of glass substrate will only become momentarily around 300 even in using a glass substrate, and then the heat damage to the glass substrate can be beside the question.
The above method to conduct a doping by an irradiation of excimer laser beam does not cause heat damage to a glass substrate. It, therefore, can prevent the substrate from being defective by the heat damage. But it poses a problem that a doping efficiency will be lowered, as an energy will not be absorbed in a reactive gas according to the wave length of laser beam. For instance, in the using of the alexandrite laser beam (wave length 745 nm), PH3 gas can not be decomposed directly. Also, in the using of AsH3 and PH3 containing pentavalent impurity, or B2H6 containing trivalent impurity, as a reactive gas for a doping (doping gas), each gas of them varies in its absorbing wave band. So that, in case where the doping of different element has to be effected, by using various kind of reaction gases, there arises a problem that the doping concentration will be disproportioned.
For example, in case a complementary type device composed of P-channel type TFT (hereinafter referred to as PTFT) and N-channel type TFT (hereinafter referred to as NTFT) is formed, or in case CMOS device is formed, it is needed to use separately each doping gas of N-type providing one and P-type providing one. Therefore, this has been a problem that the doping gas is restricted owing to a sort of laser, and the laser suitable for each doping gas should be respectively prepared.
Accordingly, it is an object of the present invention to solve the lowering of doping efficiency, which is caused by a reason that the same laser beam will not decompose a different doping gas, in case where a plurality of dopings are effected by employing different doping gases, in the doping technology through a gas phase using the above stated laser beam, in particular using the excimer laser beam.
In order to realize the above object to solve the problem in the conventional impurity doping, the following methods are proposed according to the present invention.
(1) A method for forming a semiconductor device comprising setting up a mask on a surface of semiconductor being placed in an atmosphere containing an impurity which gives one conductivity type, and diffusing said impurity into the partial region of the semiconductor, by an irradiation of a laser beam (laser light) to the surface of a semiconductor through said mask in said atmosphere, thereby making it possible to decrease the resistance rate of said region.
(2) A preparing method for a semiconductor in a selective doping method with an impurity giving a different conductivity type, comprising the process steps of: (a) Setting up a mask on a surface of semiconductor being placed in an atmosphere containing an impurity which gives P-type or N-type conductivity; (b) diffusing said impurity giving the one conductivity type into the first region of semiconductor by an irradiation of laser beam toward the surface of semiconductor through said mask; and (c) changing the later atmosphere of said process (b) into an atmosphere containing the impurity which gives N- or P-type conductivity, and also changing the position of the mask, through which a laser beam being irradiated toward the surface of semiconductor, thus making it possible to diffuse the impurity which gives N- or P-type conductivity into the second region of semiconductor.
In one example of the present invention, the mask comprises a transparent plate, e.g. a quartz plate, and a metallic pattern formed thereon. In this case, a transparent part of the mask is a part of the transparent plate on which a material of the metallic pattern is not provided. The laser light is irradiated to the surface of the semiconductor through the transparent part of the mask. The transparent part may be located on a semiconductor region of the semiconductor and a gate electrode provided on the semiconductor region during the irradiation as illustrated in FIGS. 4(A) and 4(B). In this case, the laser light reaches a surface of the gate electrode and a surface portion of the semiconductor region through the transparent part during the irradiation, and a portion of the semiconductor region is not irradiated with the laser light under the gate electrode during the irradiation. Source and drain regions are then formed in the semiconductor with a channel region therebetween under the gate electrode by the diffusion of the impurity. In another example of the present invention, the mask is a metallic mask pattern having an opening. In this case, the laser light is irradiated to the surface of the semiconductor through the opening.
In the above stated present invention, as the impurity which gives one conductivity type, it is a trivalent element and a pentavalent element respectively, in compliance with P-type and N-type of a silicon semiconductor. As the atmosphere containing an impurity which gives one conductivity type, if the conductivity is P-type, it is usually possible to use B2H6 of a reactive gas containing B (boron), a trivalent impurity which gives P-type conductivity. And if the conductivity is N-type, it is usually possible to use PH3 of a reactive gas containing P (phosphorus), a pentavalent impurity which gives N-type conductivity.
As the semiconductor, it is general to use a silicon semiconductor, but it is possible to use other ones. The fundamental point of the present invention, wherein the impurity is diffused into a semiconductor by an irradiation of laser beam, in an atmosphere containing an impurity element to be doped with, is not restricted by a kind of semiconductor. Also, as the crystal structure of semiconductor, it is needless to say that a single crystal and a non-single crystal can be used.
As the mask, it is appropriate to use a pattern formed on a quartz plate, using such a high melting point metal as chrome. The quartz plate is necessary for the transmittance of laser beam, and the pattern forming with a high melting point metal is aimed not to melt the pattern. Then, the mask may be formed using aluminum etc. if the energy density of laser beam is low. As the laser beam, XeF excimer laser (wave length, 351 nm), ArF excimer laser (wave length, 193 nm), KrF excimer laser (wave length, 248 nm) and the like can be used. As the kind of laser, it is suitable to use a pulse oscillating type excimer laser, which is of a high peak power and is capable of melting and solidifying the non-irradiated surface within a very short time.
The present invention utilizes a phenomenon that the impurity element contained in an atmosphere will diffuse instantaneously through the molten surface of a semiconductor into the semiconductor, by an irradiation of laser beam. And as the other feature of the present invention, it is indicated that both the doping and the acceleration of impurity can be performed at the same time, and further, a polycrystallization of an amorphous semiconductor can be simultaneously executed, in case the semiconductor is an amorphous one.
The present invention utilizes the above stated phenomenon, conducts selectively the doping of impurity, and further carries out continuously and selectively the doping of different impurity without a progress of photolithography. By using the present invention, it is possible to prepare CMOS type semiconductor device, which is complementarily composed of P-channel type and N-channel type MOS transistor, in a more simple process than the conventional one.
Also, the present invention has been made in the light of solving the above mentioned object, and proposes a method for doping a semiconductor with an impurity which gives one conductivity type, by an irradiation of laser beam (laser light) to the surface of semiconductor, in an atmosphere of reactive gas containing the one conductivity type impurity. Then, the present invention comprises applying an electromagnetic energy to the atmosphere during the above irradiation of laser, in order to decompose the above reactive gas. Further, it comprises heating a semiconductor which is to be doped, at the crystallization temperature of semiconductor or less, when the laser beam is irradiated.
As the impurity to give one conductivity type, a trivalent impurity, typically B (boron) etc. can be used, if it gives P-type in case of a silicon semiconductor. Conversely, a pentavalent impurity, typically P (phosphorus) or As (arsenic) etc. can be used, if it furnishes N-type. And as the reactive gas containing such an impurity, AsH3, PH3, BF3, BCl3, B(CH3)3, and the like can be used.
As the semiconductor, in case of the preparation of TFT, an amorphous silicon semiconductor thin film, which is formed by a gas phase growth method or by a sputtering method, is generally used. A polycrystal or a single crystal silicon semiconductor, which is prepared by a liquid phase growth method, can also be used in the present invention. Further, it is needless to say that other semiconductors can be used, not being limited to the silicon semiconductor.
As the laser beam, it is useful to employ a pulse oscillating type excimer laser apparatus, because a heat damage to the substrate is substantially none at all in case of the excimer laser as aforementioned, and there has been an actual result that a polycrystal thin film with a high crystalline property can be obtained, in case an amorphous silicon semiconductor is crystallized by an irradiation of laser.
As an actual kind of laser, it is suitable to use ArF excimer laser (wave length, 193 nm), XeF excimer laser (351 nm), KrF excimer laser (248 nm) and the like.
As the electromagnetic energy which is applied to the decomposition of the reactive gas for a doping (doping gas), 13.56 MHz of high frequency energy is generally used. In case where a laser beam, which can not decompose the doping gas directly, is used, it is possible to efficiently execute a doping, by decomposing the doping gas with this electromagnetic energy. The kind of electromagnetic energy is not limited to the frequency of 13.56 MHz, but a still higher activation rate can be accomplished by using e.g. 2.45 GHz of microwave. Further, it may be possible to use ECR condition which is generated by a mutual action of 2.45 GHz of microwave and 875 gauss of magnetic field. And it is effective to use a light energy which is capable of the direct decomposition of doping gas.
The heating of the semiconductor sample, at a crystallization temperature of semiconductor or less than it, in the laser irradiation is intended to promote the activation of impurity in a semiconductor and the crystallization of a semiconductor, by lengthening the cooling rate of the sample which is momentarily heated by laser beam and by effecting a dehydrogenation of the sample. The crystallization temperature of a semiconductor is the temperature wherein the semiconductor is transformed from an amorphous to a crystal state (in general, microcrystal or polycrystal state), and it is 500 to 550xc2x0 C. in case of silicon.
This heating at the crystallization temperature or less is resulted from the reason that if a crystalline semiconductor is doped by heating, at a higher temperature than its crystallization temperature, there will occur a controlling trouble of valency electron caused by a generation of levels. Also, in case of TFT using an amorphous silicon (indicated as a-Si: TFT), if it is heated at 350xc2x0 C. or more according to the present invention, the device will be broken down. Therefore, it is suitable to heat at 350xc2x0 C. or less in this case.